RESOLVING THE RIFTING OF RODINIA: A NOVEL COMPARATIVE DETRITAL GEOCHRONOLOGY APPROACH APPLIED ALONG THE FULL LENGTH OF THE APPALACHIAN–CALEDONIAN SYSTEM

A key aspect of plate tectonics which remains poorly understood is how rifting initiates. The Appalachian–Caledonian system (AppCal)—a natural laboratory to assess how tectonic regimes switch and mountain belts evolve—records Neoproterozoic–Cambrian rifting of Rodinia to form the Iapetus Ocean, with widespread evidence for distinct thermotectonic events at c. 760–700 Ma, c. 600 Ma and c. 540 Ma. This study develops and applies a novel comparative detrital geochronology approach to investigate the tempo, length- and timescales of thermotectonism associated with Rodinian rifting. The approach juxtaposes tectonothermal histories in detrital mineral systems that record age signals over different length scales of sediment transport; white mica, being more susceptible to weathering and thermal overprinting than zircon, should be more locally-derived than zircon, and thus should better capture the spatial distribution of episodic Neoproterozoic–Cambrian thermotectonism.

Greenschist-facies metasediments (T_peak < 400 °C) were collected from three sampling clusters along AppCal—Southern App (TN–VA), Northern App (NY–VT–QC) and Scottish Cal—representing ~4000 km of along-strike rift–drift deposition during the opening of the Iapetus Ocean. Electron probe microanalysis (EPMA) performed on 720 white mica grains across 23 rocks reveals a large range in detrital white mica composition from each rock/cluster, suggesting limited post-depositional recrystallization (chemical homogenization); detrital white mica geochemistry should be controlled by thermotectonic source, meaning that compositional variation in Al, Mg, Fe, etc. may reflect the mosaic of distinct lithotectonic sources that supplied detritus to each rock/cluster. ⁴⁰Ar/³⁹Ar white mica and U–Pb zircon age signatures in the same rocks provide the opportunity to resolve more subtle and local fingerprints from more far-travelled signatures of rift histories along AppCal. Localized thermal activity that is asynchronous along AppCal may support bottom-up (plume-driven) origins for episodic thermotectonism during AppCal rifting. Alternatively, thermotectonic episodes that were synchronous along the entire AppCal may support a rifting mode dominated by top-down (far-field stress) processes.